1. Field of the Invention
The invention relates to the field of electro-optical modulators and in particular modulation of an optical carrier with electrical signals in the GHz range.
2. Description of the Prior Art
Fiber optic links are important in a wide variety of applications such as millimeter wave communications and radar systems. An external electro-optic modulator is usually required for a millimeter wave fiber optic link, since direct modulation of a solid state laser signal is generally not possible above microwave frequencies. Traveling wave integrated optic modulators used for this purpose are known in the art, such as described in a paper entitled “17 GHz bandwidth electro-optic modulator”, by C. Gee et al, in Applied Physics Letters, vol. 43, no. 11, Dec. 1, 1983, pp. 998-1,000. A typical traveling wave modulator includes a substrate formed of an electro-optic material, preferably crystalline lithium niobate (LiNbO2). An optical waveguide is formed in the substrate just below the surface of the crystal by ion diffusion of titanium or proton exchange. The waveguide is single mode, and typically only a few microns wide. An optical signal from a laser or the like is fed into an input and retrieved from an output of the waveguide using focusing lenses or by close coupling to single mode optical fibers. A microwave strip line electrode is deposited on the surface of the substrate immediately adjacent to the optical waveguide. An electrical signal at microwave or higher frequency is applied across the segments of the strip line electrode through a coaxial cable. The electrode is terminated in a resistive load via a coaxial cable. The electrical signal applied to the electrode through the cable propagates along the electrode parallel to the optical waveguide as a traveling wave.
The segments are sufficiently small and close together that the transverse electric field therebetween resulting from the electrical signal propagating along the electrode passes through the optical waveguide and induces an incremental phase shift in the optical signal via the electro-optic effect. This incremental phase shift is integrated along the length of the optical waveguide to produce the net phase modulation. The optical waveguide can be split into two branches in a Mach-Zehnder type interferometer arrangement to provide amplitude modulation as described in the above referenced article to Gee et al.
The integrated effect of the incremental phase shift is cumulative as long as the optical and electrical signals propagate parallel to each other at the same phase velocity. However, this does not occur in practical electro-optic materials such as LiNbO2. At optical frequencies, the refractive index of LiNbO2 is no=2.2, whereas at microwave and millimeter wave frequencies the refractive index is nmm=5.3 to 6.6, depending on the orientation (LiNbO2 is anisotropic). Since the electric field between the segments of the strip line electrode passes through both air and LiNbO2, the effective index of refraction for the electrical signal traveling along the electrode is on the order of neff=4. This is still a mismatch with the no=2.2 for the optical signal.
Due to the refractive index mismatch, the optical signal propagates with a phase velocity, which is approximately twice that of the electrical signal. The magnitude of the phase modulation progressively decreases as the phase difference between the optical and electrical signals increases. This phenomenon is known as a phase “walk off”. The decrease in overall phase modulation with frequency f and interaction length L is equal to [(sin(AfL))/AfL]2, where A=2π/c(neff−no), and c is the speed of light.
This velocity mismatch necessitates design tradeoffs. The maximum achievable drive electrical drive signal frequency f decreases as the interaction length L is increased. Conversely, to lower the drive voltage and power, a long interaction length L is required. The modulator must be made shorter and the drive power larger as the frequency is increased to obtain satisfactory modulation.
Prior art attempts to compensate for this phase velocity mismatch include replacing the single electrode with a periodic electrode structure such as described in a paper entitled “Velocity-matching techniques for integrated optic traveling wave switch/modulators”, IEEE Journal of Quantum Electronics, vol. QE-20, no. 3, March 1984, pp. 301-309. These periodic electrode structures can be categorized into either periodic phase reversal or intermittent interaction electrodes. Known intermittent interaction electrode configurations include unbalanced transmission lines, i.e., asymmetric about the propagation axis. This leads to incompatibilities with the balanced line (typically coaxial or waveguide probe) transitions to other fiber optic link transmitter components.
The periodic phase reversal structures break up the electrode into shorter sections, and force the phase shift between the sections to match the optical phase shift. The electrode is assumed to consist of four sections, with a 180 degree phase shift between the individual sections. The relative phase of the optical and electrical signals is effectively reset at the leading or upstream end of each section, and deviates to a maximum extent, which is inversely proportional to the length of the sections. Thus, the phase velocities are matched on the average. However, there is still a reduction in the modulation by the factor [sin(AfLsection)/AfLsection]2, and Lsection is required to be long enough to produce a 180 degree phase delay. This also means that the 180 degree phase reversals are correct only at a single modulation frequency, so that the low-pass modulator is converted into a bandpass modulator.
Other problems that make it difficult to extend the operation of such modulators to millimeter wave or higher frequencies, involve the connection of modulation electrodes to the modulation signal source by coaxial cables, or through wire bonds or the like. This becomes unmanageable due to the extremely small physical dimensions involved.
Lithium niobate modulators for 40 GHz will require some means of countering velocity mismatch in order to retain sufficient modulation sensitivity. Several solutions are possible:
A first approach of the prior art is called “true” velocity matching. The phase velocity of the RF transmission is made to equal the optical velocity in the optical waveguide by adding a thick silicon dioxide buffer layer, and up-plating the transmission line conductor so that it extends significantly into the air above the chip. Both of these actions, pull the electric field out of the lithium niobate, thus increasing the phase velocity. Unfortunately, this also reduces the sensitivity. The argument that this loss in sensitivity can now be made up with a longer modulator is countered by the increasing electrode loss at EHF ranges.
A approach of the prior art is called velocity matching “on the average” by segmenting and rephasing the RF transmission line, for example by phase reversal or adding phase delay to match on a space harmonic. This scheme results in a band-pass structure and still is limited by electrode loss.
A third approach of the prior art is called velocity matching “on the average” by segmenting the modulator's RF transmission line and then feeding the segments with a “corporate feed” comprised of branched RF transmission lines which introduce true time delay to the segments. See U.S. Pat. Nos. 5,291,565 and 5,076,655, which are both incorporated herein by reference. In this case, the modulator remains a low-pass device. The electrode loss is counteracted since the loss per segment is now only αL/N rather than αL, where α is a loss factor, L is an interaction length and N is the number of segments. Of course, the input power is divided by N so that a penalty of √N is incurred. The trade-off to find the optimum value of N for a given value of α is discussed in the chapter by Bridges entitled “Antenna Coupled Millimeter-Wave Electro-Optical Modulators,” in RF Photonic Technology in Optical Fiber Links, W. S. C. Chang, Editor, Cambridge University Press 2001. An additional penalty is imposed by the loss in the corporate feed itself, which is why U.S. Pat. Nos. 5,291,565 and 5,076,655 propose that the corporate feed be located on a separate low-loss ceramic substrate with optimized conductor widths and thicknesses.
A fourth approach is called velocity matching “on the average” by segmenting the modulator's RF transmission line as in the third approach discussed above, but then using free space propagation at an angle to provide the true time delay, then coupling to the segments by on-chip antennas. This avoids the added transmission line loss from the corporate feed, but now is limited by the bandwidth of the antennas. The third approach would seem to be a good one for digital lithium niobate modulators for 40 GHZ and above. The optimum value of N can be determined from the curves present in RF Photonic Technology in Optical Fiber Links referenced above, or the earlier discussion in Finbar T. Sheehy's Ph.D. thesis referenced therein.